LabVIEW
Using a 2-D array Single Process Shared Variable w/ RT FIFO for comm between a Deterministic and non-deterministic loop on an RT Target
Our problem is that we currently use a 2D array to store CAN data on a Real-time Target. The array is 20 elements of 3 byte elements as so:
0 1 2
0 [byte] [byte] [byte]
…
19 [byte] [byte] [byte]
These values are passed between a Deterministic Timed (DT) loop where they are set and a Non-Deterministic Timed (NDT) loop where they are read and passed into a Network Published Shared Variable (NPSV) for communication across the network to a Host PC. I have insrted an image for illustration, pardon the size.
Currently to pass the data between the DT and NDT loop we are using a Global Variable (GV). To improve the system we have attempted to replace these GVs with Single Process Shared Variables (SPSV) with an RT FIFO enabled.
To create the shared variable I simply right clicked the GV of interest and selected create Shared Variable Node form the drop downs. At this point LabVIEW presented me with a 2D NPSV within a new Library hosted on the RT Target. I then selected this new NPSV from the Project, changed it to a SPSV, and enabled a single element FIFO. This variable was initialized with a default value for the size described above and then used in our code for the DT to NDT communication, and conversion to a corresponding NPSV for sending to the Host.
When I went to run the code I noticed that the variable was in fact 2D, however its size was only 2 elements of three bytes each, in other words only two of the row indices were populated and the other appeared as uninitialized. in addition, this data had no resemblance to the set initilazation value. This was also how the variable was presented on the host side of the network after tranfer into a NPSV.
The peculiar part is that If I change this SPSV to a NPSV and then try to change it back, I receive an error saying the type is not supported for SPSV with an RT FIFO enabled. I have to disable the FIFO (which defeats the entire purpose) in order to successfully compile! I am unclear as to what is the bug in this case. Should I not be allowed to create the original 2D SPSV with a single element RT FIFO enabled without receiving an error? Or if this is okay how do I fix the problems associated with the variable after being allowed to create it?
I have found the following discussion in which a user states “The only limitations for custom controls is the ability to use it with RT FIFO enabled on a network-published shared variable”. Is this also true for SPSV? I have not found any documentation explicitely stating this for SPSV, though it is stated for the NPSVs.
